refactor(host)!: retire the skill.invoke RPC for the gesture boundary

Invocation is an ordinary session.prompt again: the pre-step gesture
boundary makes it deterministic host-side for every front end, so the
dedicated RPC (handler, wire schema, error codes, client face, fixtures)
and ui-skill's claim machinery are net deletions. The menu keeps decision
21 exactly — a pick lands literal /name text — plus the user-only marker
from skill.list's modelInvocable flag.

packages/client/runtime/src/client/index.ts

@@ -45,12 +45,11 @@ export { createSnapshotStore, defineStore, shallowEqual } from './contract/store
 export type {
   EngineStoreHandle, EngineStoreInstance, ObservableSnapshot, SnapshotStore,
 } from './contract/store.ts'
-export { opensUserTurn } from './sessions/conversation.ts'
 export type {
   AssistantBlock, AssistantMessageNode, AssistantProvenanceView, AssistantRequestConfig,
   AssistantTiming, CodeSubCall, CommandNode, CompactionSummaryNode, ComposerPhase,
   ContextMessageNode, ConversationNode, ConversationSnapshot, ModelRetryNode, QueuedMessage,
-  RunningToolCall, SkillInvocationNode,
+  RunningToolCall,
   SteeringMessageNode, TodoItem, ToolResultNode, TurnErrorNode, UnknownSurfaceNode, UserMessageNode,
 } from './sessions/conversation.ts'
 export type {

packages/client/runtime/src/client/sessions/context-provenance.ts

@@ -83,6 +83,9 @@ export function contextProvenance(source: unknown): ContextProvenanceView {
       return { role: 'inject', label: joined(collect(record, 'changes', 'path')) ?? kind }
     case 'plugin':
       return { role: 'inject', label: readString(record, 'plugin') ?? kind }
+    // A user-explicit skill invocation names the skill it injected.
+    case 'skill-invocation':
+      return { role: 'inject', label: readString(record, 'name') ?? kind }
     // Documented default arm of the merge-extensible source map: an unknown
     // producer still identifies itself by its own durable kind.
     default:

packages/client/runtime/src/client/sessions/conversation.ts

@@ -129,25 +129,6 @@ export interface ContextMessageNode {
   form: KnownContextForm | null
 }
 
-/**
- * A user-explicit skill invocation: the host injected the rendered skill as a
- * user message carrying the `skill-invocation` source, so the card presents
- * `/name args` from source metadata and collapses the injected body.
- */
-export interface SkillInvocationNode {
-  kind: 'skill-invocation'
-  seq: number
-  /** Unix epoch ms from the source session event. */
-  time: number
-  /** Invoked skill name read off the message source. */
-  name: string
-  /** Trailing user text read off the message source, when recorded. */
-  args?: string
-  /** Full injected model-facing content (collapsed by default in the UI). */
-  content: readonly ContentBlock[]
-  source: unknown
-}
-
 /** Durable notice that a closed failed step is waiting for a model-request retry. */
 export type ModelRetryNode = LlmRetryEventData & {
   kind: 'model-retry'
@@ -258,27 +239,12 @@ export interface CommandNode {
   outcome: { kind: 'success' | 'error'; text?: string } | null
 }
 
-/**
- * Whether a node opens a user turn on the transcript surface. A direct user
- * message and a user-explicit skill invocation both start the turn the next
- * assistant answer closes; parallel consumers (turn boundaries, retry
- * liveness, own-words scrolling) share this one predicate instead of each
- * re-encoding the kind list. Steering stays out: an interjection lands
- * mid-turn and closes nothing.
- * @param node - any conversation node.
- * @returns true for the user-turn-opening kinds.
- */
-export function opensUserTurn(node: Pick<ConversationNode, 'kind'>): boolean {
-  return node.kind === 'user' || node.kind === 'skill-invocation'
-}
-
 /** Finalized conversation node union (kind discriminates; seq is the React key). */
 export type ConversationNode =
   | UserMessageNode
   | AssistantMessageNode
   | SteeringMessageNode
   | ContextMessageNode
-  | SkillInvocationNode
   | ModelRetryNode
   | TurnErrorNode
   | ToolResultNode

packages/client/runtime/src/client/sessions/transcript-adapter.ts

@@ -58,22 +58,10 @@ function materializeNode(
 ): ConversationNode {
   switch (event.type) {
     case 'user/message': {
-      // A user-explicit skill invocation carries its name (and optional args)
-      // on the source; the dedicated node lets the card render `/name args`
-      // from metadata instead of re-parsing the injected body. A record whose
-      // name is unreadable degrades to injected context below.
-      const source = event.data.source as { kind?: unknown; name?: unknown; args?: unknown }
-      if (source.kind === 'skill-invocation' && typeof source.name === 'string') {
-        return {
-          kind: 'skill-invocation', seq: event.seq, time: event.time,
-          name: source.name,
-          ...typeof source.args === 'string' ? { args: source.args } : {},
-          content: event.data.content, source: event.data.source,
-        }
-      }
-      // Injected context (plugin/goal source) folds to a context node, not a
-      // user message; only a direct human prompt is a user node. A compaction
-      // checkpoint never reaches here (isCompactCheckpoint routes it away).
+      // Injected context (plugin/goal/skill-invocation source) folds to a
+      // context node, not a user message; only a direct human prompt is a
+      // user node. A compaction checkpoint never reaches here
+      // (isCompactCheckpoint routes it away).
       if (event.data.source.kind !== 'user') {
         return {
           kind: 'context', seq: event.seq, time: event.time,